Patients suffering from weakness of the muscles of the thoracic cage and diaphragm, as may occur in, for example, Duchenne Muscular Dystrophy or Cervical Spine Injury, are often unable to cough effectively, if at all. Due to their inability to clear respiratory secretions from their lower respiratory tract, retained secretions may develop in their lungs. As retained secretions constitute a focus for infection, these patients are at high risk of developing severe, and potentially fatal, pneumonia, and are thus in need of assistance in expectorating their respiratory secretions.
Several techniques for assisting such patients are known. If the degree of muscle weakness is mild, physical therapy techniques such as “manually assisted coughing” may be effective. With this technique, the therapist enhances the efficacy of the patient's natural cough by means of a hand thrust on the patient's ribcage or abdomen, timed in coordination with the patient's natural coughing action. If the degree of muscle weakness is severe, however, the patient will require mechanical assistance to achieve adequate pulmonary toilet.
For patients who are intubated with an endotracheal tube, or have a permanent tracheostomy cannula in place (either of which may be necessary for purposes of mechanical ventilation due to the severity of the chest wall muscle weakness), endotracheal suction is commonly used as a technique for secretion clearance. Endotracheal suction is achieved by inserting a narrow gauge catheter into the patient's trachea via a larger gauge endotracheal tube or tracheostomy cannula, and then applying suction through the catheter. Secretions that are in proximity to the tip of the catheter are then sucked into the catheter and removed. This technique achieves secretion removal by utilizing a suction force to cause secretions to either adhere to the catheter tip or enter into the catheter, which is then withdrawn from the body while the suction force is maintained. It should be noted that the suction is generated within the suction catheter only, not within the endotracheal tube or tracheostomy cannula, and that it is executed at any stage during the patient's respiratory cycle, be it inspiration or expiration.
There are several drawbacks to endotracheal suction as a means for clearing respiratory secretions. The procedure is invasive, thus requiring sterile technique for its performance, and may cause physical trauma to, or infection within, the patient's airways. Moreover, this technique can only be performed on those patients who are already intubated or tracheostomized, and is not relevant to the majority of patients who do not have instrumentation within their respiratory tract.
For non-intubated patients, and for intubated patients who wish to avoid the above-mentioned drawbacks of endotracheal suction, a desirable mechanical method for removal of tracheobronchial secretions is that of mechanical insufflation-exsufflation (also known as inexsufflation), by means of an inexsufflator. The Concise Oxford Dictionary, Seventh Edition, Oxford University Press, 1985, defines insufflation as “blowing air into a cavity of the body”, and Blackiston's Gould Medical Dictionary, Fourth Edition, McGraw-Hill, 1979, defines exsufflation as “forcible expiration; forcible expulsion of air from lungs by a mechanical apparatus”.
Most commonly, an inexsufflator is applied to a patient's respiratory tract via a facemask held hermetically over the patient's mouth and nose, a nasal mask, nasal prongs, or a mouthpiece held in the patient's mouth. All of the aforementioned are hereinafter referred to as “noninvasive ventilation interfaces”, by which is meant a ventilation interface that does not penetrate into the patient's trachea, but rather interfaces with the patient's mouth and/or nose. Alternatively, if the patient is intubated or has a tracheostomy, the inexsufflator may be attached directly to the endotracheal tube or tracheostomy cannula (which are “invasive ventilation interfaces”). Typically, an inexsufflator functions in a cyclical fashion as follows: First, the inexsufflator mechanically pumps air into the patient's lungs until the lungs have expanded to their maximum capacity (insufflation). Then, at the moment of peak insufflation, the inexsufflator rapidly sucks air out of the patient's lungs at a high flow rate. This rapid flow of air through the patient's respiratory tract outward (exsufflation) carries with it secretions from the lower respiratory tract. In this manner, an inexsufflator artificially simulates the action of a natural cough.
Thus, in contrast to endotracheal suction, inexsufflation is noninvasive (and thus does not require sterile technique or cause trauma to the airways), and achieves secretion removal by causing rapid airflow (at least 160 liters/minute) through the entire respiratory tree, as occurs during a physiological cough. This airflow “blows” the secretions up the trachea and into the patient's mouth (or tracheostomy cannula/endotracheal tube if the patient is intubated).
Many patients in need of an inexsufflator have weakness of their facial and glossopharyngeal muscles in addition to the weakness of their chest wall muscles, and they typically are unable to “hold in” a deep breath for a significant period of time. As such, after an inexsufflator completes the lung insufflation cycle, the insufflated air may rapidly dissipate through the patient's mouth and nose. It is thus of critical importance for the successful functioning of an inexsufflator that the cycle of mechanical exsufflation commences immediately after full insufflation has been achieved, prior to air dissipation, because if the onset of exsufflation is even marginally delayed the volume of air within the patient's lungs which will be available for mechanical exsufflation will be significantly diminished, resulting in an ineffective “cough”.
Mechanical inexsufflation is particularly effective when it is augmented by the manual assisted cough technique described above. This is usually done by a single caregiver (often a physiotherapist or a member of the patient's family) who operates the inexsufflator while simultaneously applying abdominal/chest thrusts timed to the exsufflation phase of the machine.
It is physiologically desirable that each insufflation-exsufflation cycle be separated from the preceding or following cycle by a pause of at least a few seconds, so as to prevent hyperventilation of the patient. During this expiratory pause period no airflow is generated by the inexsufflator, such that the intrapulmonary pressure equilibrates to atmospheric pressure (that is, zero) during this time, until the onset of the next insufflation.
Standard inexsufflators, such as the CoughAssist Inexsufflator (J. H. Emerson Co. Cambridge, Mass.) utilize a blower to generate airflow within the machine. This mechanism is used to generate airflow alternately in two directions: into the lungs under positive pressure during insufflation, and out of the lungs under negative pressure during exsufflation. In the “automatic” version of this machine, model CA-3000, cycling from insufflation to exsufflation is achieved by means of an electrically operated switching mechanism that automatically redirects the direction of airflow between the patient and the machine (either into or out of the blower) according to a predefined time sequence. Alternatively, the operator can control the timing of the insufflation-exsufflation cycles by pushing an electric switch in a respectively rightward or leftward direction while the machine is operating, in accordance with the time sequence desired by the operator.
The electrical timing mechanism within automatic inexsufflators, and the mechanism for generating positive-pressure airflow into the patient, makes these devices both electronically complex and expensive. The cost of such devices (approximately $5000 in 2002) is of particular importance because many patients in need of an inexsufflator are concurrently in need of a similarly expensive mechanical ventilator for purposes of mechanical ventilation via an invasive or noninvasive ventilation interface, as their chest wall muscle weakness limits not only their ability to cough, but also their ability to breath adequately and independently. Such patients, who are often already using a mechanical ventilator, are thus compelled to acquire an additional expensive ventilatory device if they wish to perform mechanical inexsufflation.
So as to decrease the cost of standard inexsufflator devices, “manual” versions of the CoughAssist Inexsufflator have been developed, such as the CoughAssist model CM-3000 (J. H. Emerson Co. Cambridge, Mass.), which does not have an electrical timing mechanism. In this manual version, the operator manually controls the timing of the insufflation-exsufflation cycles by swiveling or rotating a mechanical non-electronic handle to-and-fro in a respectively rightward or leftward direction, so as to mechanically redirect the direction of patient airflow into or out of the blower.
Standard inexsufflators, however, are known to suffer from several deficiencies:
1) The equilibration of intrapulmonary pressure with atmospheric pressure that occurs during the expiratory pause phase of the respiratory cycle impedes effective secretion clearance. This is because the lack of positive (i.e. supra-atmospheric) intrapulmonary pressure during this period encourages the collapse of smaller airways and alveoli, which traps secretions deep within the lung.
2) Standard inexsufflators are comprised of a built-in mechanism for generating airflow in two directions (both into and out-of the patient's lungs), which is mechanically complex. As such, standard inexsufflators (both manual and automatic versions) are expensive, with even manual inexsufflators costing approximately $3000 in 2002.
3) It is difficult for a caregiver performing the manual assisted cough technique simultaneously with mechanical inexsufflation to achieve optimal coordination with a manual inexsufflator. This is because each of the caregiver's hands have to perform a different maneuver simultaneously: while one hand has to perform an “in-out” abdominal/chest thrust on the patient, the other hand has to perform a “side-to-side” rotary movement of the swivel handle on the manual inexsufflator. Precise coordination of left and right hand movements, which is essential for achievement of effective cough flows, is particularly difficult when each hand is performing a different gross motor movement.